1. Field of the Invention
The present invention relates to a magnetic force microscope (MFM) that is a type of scanning probe microscope (SPM) and acts to image the magnetic field distribution on the surface of a sample.
2. Description of the Related Art
FIG. 1 shows a conventional magnetic force microscope. In this instrument, an oscillator 102, such as a function generator, is set to produce an output signal having a frequency close to the resonance frequency of a cantilever 101. This output signal from the oscillator 102 is supplied to a piezoelectric element 103 for exciting the cantilever 101 into resonance oscillation. The cantilever 101 is forced to oscillate at that frequency.
The rear surface of the cantilever 101 is illuminated with laser light from a laser light source 104. Light reflected from the cantilever is detected by an optical detector 105 such as a quadrant photodiode. Oscillatory deflection of the cantilever 101 is detected by this optical detector. The output signal from the optical detector 105 is electrically amplified by a preamplifier 106 incorporating a band-pass filter.
The output signal from this preamplifier 106 is sent to an amplitude/dc voltage converter (RMS-DC) 107, which detects the amplitude of the deflection of the cantilever 101. Z direction (vertical) motion of a piezoelectric element scanner 111 is so controlled by an error amplifier 108 via a filter 109 and a z piezoelectric driver power supply 110 that the amplitude is maintained constant.
The amplitude kept constant by the error amplifier 108 is set by a reference voltage-setting means 112 to such a value that the probe of the cantilever 101 taps a sample 113. In this tapping mode, the oscillation of the cantilever 101 reflects the topographic information about the sample more strongly than magnetic information.
Under this condition, the output signal from the filter 109 that controls the z motion corresponds to the topography signal from the surface of the sample 113. Therefore, x- and y-scan signals are supplied to the piezoelectric element scanner 111 to scan it in two dimensions. A topographic image is obtained by using the z motion control signal.
Magnetic force imaging of the sample 113 is next described. The trajectories of scans made by the piezoelectric element scanner 111 (i.e., variations in z motion versus x and y scans) in the topographic imaging mode are stored in a memory 114. During magnetic force imaging, a switch S1 is switched to the side of the memory 114. The piezoelectric element scanner 111 is so controlled based on the z position information stored in the memory 114 that a scan is again made across a position lower than the previous z position by a given amount xcex94z.
The phase difference between the oscillation signal (reference signal) from the oscillator 102 for exciting the cantilever into resonance oscillation and the output signal from the preamplifier 106 indicating the actual oscillation of the cantilever at this time is detected by a phase detector 115. The output signal from this phase detector is used as an MFM signal. In this way, a magnetic force image of the sample 113 is obtained.
During magnetic force imaging, the distance between the cantilever and the sample is set greater than during topographic imaging and the cantilever is kept out of contact with the sample as mentioned previously. In this non-contact mode, the oscillation of the cantilever reflects magnetic information about the sample more strongly than topographic information. Consequently, a magnetic force image free of the effects of topography of the sample can be obtained. The aforementioned topographic image and magnetic force image can be alternately obtained for each pixel, line, or frame.
The above-described magnetic force imaging is generally performed within atmosphere. If magnetic force imaging is performed within atmosphere by the use of the magnetic force microscope of FIG. 1, then a magnetic force image free of the effects of topography of the sample can be derived in a relatively short time.
However, where a method generally known as the slope detection method (i.e., topographic imaging is performed with the instrument of FIG. 1 in a vacuum), the Q value of the cantilever is quite large, as described by Albrecht T. R., Grxc3xcitter P., Home D., and Rugar D.; J Appl. Phys., 69, 668 (1991). The cantilever easily oscillates. The result is that the responsiveness of amplitude variation deteriorates greatly. That is, the frequency range of the output signal from the amplitude/dc voltage converter 107 of the instrument shown in FIG. 1 narrows greatly.
Therefore, where a topographic image of the sample placed in a vacuum should be obtained by the magnetic force microscope of FIG. 1 using the slope detection method, the scan speed of the piezoelectric element scanner 111 needs to be made lower by one or more orders of magnitude than in imaging within atmosphere.
Furthermore, we have discovered that an accurate topographic image of a sample placed in a vacuum cannot be obtained by the use of the slope detection method, even if the scan speed is decreased as mentioned above.
This is described in further detail. If the sample is imaged within atmosphere with the instrument of FIG. 1, the reference voltage-setting means 112 is adjusted to reduce the amplitude of the cantilever to some extent. Under this condition, the probe tip of the cantilever intermittently touches, or taps, the sample surface. In this state, the amplitude of the cantilever is little affected by magnetic force.
In a vacuum, however, the amplitude of the oscillating cantilever is dominated by the effects of shift of the resonance frequency of the cantilever that is the principle of the non-contact atomic force microscope (NC-AFM). In consequence, the tapping mode cannot be accomplished during topographic imaging. In particular, if the oscillating cantilever placed in a vacuum is brought closer to the sample, the cantilever undergoing a force easily comes out of its narrow resonant frequency range. The amplitude decreases violently though the probe of the cantilever is not tapping the sample. Therefore, the probe of the cantilever never taps the sample if the reference voltage-setting means 112 is adjusted so as to reduce the amplitude of the cantilever to some extent.
For this reason, when a topographic image of the sample is obtained in a vacuum using the instrument of FIG. 1, the topographic image contains magnetic information about the sample. As such, the separation between the topographic image and the magnetic force image is insufficient.
In the instrument of FIG. 1, the piezoelectric element scanner 111 is caused to move a distance xcex94z for each pixel or for each line to achieve magnetic force imaging as mentioned previously. This z motion will oscillate the piezoelectric element scanner 111. The oscillation will be transmitted to other components of the instrument, making the imaging unstable.
In one method, the z position of the piezoelectric element scanner 111 is kept at a constant height. Under this condition, the piezoelectric element scanner 111 is scanned in the x- and y-directions. The deflection of the cantilever occurring at this time is detected and thus a magnetic force image is obtained. In this method, the tip-sample separation differs greatly from location to location. Consequently, an accurate magnetic force image cannot be obtained.
It is an object of the present invention to provide a magnetic force microscope which can produce a topographic image containing no magnetic information if the imaging is performed in a vacuum and which assures stable topographic imaging and magnetic force imaging.
This object is achieved in accordance with the teachings of the present invention by a magnetic force microscope comprising: a magnetized cantilever having a supported end and a free end located on opposite sides; a probe attached to the free end of the cantilever; an oscillation means for exciting the cantilever into oscillation such that the cantilever oscillates at a given oscillation frequency of f0 and with a given amplitude when the cantilever is at such a distance from a sample that no force is exerted between the cantilever and the sample; a distance control means for controlling the distance between the cantilever and the sample to cause the oscillation frequency of the cantilever to shift from the oscillation frequency f0 to f1 (f1 less than f0) so that the probe taps each observation position (xi, yj) on the sample; a means for obtaining topographic information about the sample in the observation position (xi, yj) on the sample, based on results of control provided by the distance control means; a position-setting means for placing the probe in the observation position (xi, yj) on the sample and maintaining the distance between the cantilever and the probe at a distance occurring when the topographic information is obtained; an amplitude control means for controlling the oscillation means in such a way that the amplitude of the cantilever excited into oscillation by the oscillation means does not permit the probe to tap the sample when the probe has been placed in position on the sample by the position-setting means; and a means for obtaining magnetic information about the sample in the observation position (xi, yj) on the sample based on the oscillation frequency of the cantilever when the probe has been placed in position by the position-setting means and the cantilever is oscillating such that the probe does not tap the sample under control of the amplitude control means.
Other objects and features of the invention will appear in the course of the description thereof, which follows.